In high-volume xerographic printing, it has recently become of interest to provide “custom color” options, in which a dedicated developer unit dispensing toner of a very specific color is provided. A custom color is typically desired by a customer using a characteristic, and sometimes proprietary, color for letterheads and other purposes. In one business model, a printing or supplies vendor blends two or more commercially-available component color toners to obtain a mixture having the specific desired color. The mixture is then used directly in a single developer unit, along with the standard black developer unit, within a highlight-color printing apparatus or in a “fifth housing” in a full-color printing apparatus.
In a practical application of a custom color xerographic system, the use of a mixture of color toners in a single developer unit presents certain challenges. Most notably, different types of toner, such as corresponding to different component colors, may be electrostatically drawn from the developer unit at different rates:
unchecked a “faster-going” type of toner will be drawn out of the developer unit toward a photoreceptor at a high rate toward the beginning of use, leaving a high concentration of a “slower-going” toner (of a different color) in the developer unit. In short, the use of a mixture of toners can cause a drift of the actual color produced by the developer unit over time. Further, the two types of toner may have significantly different electrostatic properties, and the single developer unit may be controlled assuming a set of electrostatic properties of the predetermined original mixture: as the relative concentration of the two types of toner drifts away from that of the original mixture, electrostatic control of the unit, even with feedback control, becomes uncertain.
An optical approach for sensing toner concentration has been used in a broad range of noises and under machine operating conditions. However, for sensing of mixtures of toners of different types, using a single LED emission wavelength cannot provide color information. This sensing approach uses the fact that the color of the developer, i.e., its L*a*b* values, monotonically increases or decreases as a function of increasing or decreasing concentration of the toner mixture and of the constituents of the mixture. A spectrophotometer based device can provide the same information, but this approach has some shortcomings, such as a need for a broad band white light source, and a network of fiber optics to transmit the signal to and from the various color-toner sumps within a printing apparatus.
This disclosure proposes an optical toner concentration sensor that provides an alternative to the spectrophotometer.
The two patent documents incorporated by reference above demonstrate the overall strategy of using and controlling a custom-color developer unit with a powdered-toner mixture. U.S. Pat. Nos. 5,781,828 and 6,575,096 describe a color control system for xerography using toners suspended in liquids.